Re: [CAD_CAM_EDM_DRO] servo amps, power supplies; addl endearments

jmw@... wrote:

> Alan, Jon et al thanks for your thoughful replies to my previous post
> re servo amps and power supplies.
>
> After reading the docs for the Baldor UM4-100 amps-in-a-cage I've
> got, I'm starting to think that the power supply for the axes motors
> is built-in. i) The chassis has a transformer and cap each of which
> are half-beer can sized (12 oz, not a Foster's), ii) the specs call
> out "rated bus input nominal 70-105 VAC" and "nominal output voltage
> 100 VDC", iii) the chassis connections diagram shows "isolated AC
> input power 70/105 VAC,

Well, there it is! You need an external transformer to provide the
AC input. The rectifier and cap are there, but no transformer!

> Before doing that, however, I'd like to find out what an
> isolation transformer is.

It is a device that generally has the same input and output voltages,
but no direct electrical connection between the two, only a magnetic
connection.

> The "typical connections" diagram for the individual amp cards shows
> a tachometer with two leads going to + and - tach terminals, plus
> limit switches and interlock/reset, and something called "motion
> controller +/- 10 VDC signal" with leads going to "differential"
> and "signal input" terminals. So, what is "motion controller"? This
> sounds suspiciously like the brains of the operation, so I assume
> it's the soon to be released servo card Jon is working on.

Yes, or a servo-to-go card, or other servo CNC control.

> Where do the rotary encoders fit into this scheme? Do the encoder
> outputs run to the PC-servo card and that card sends output to the
> motion controller terminals on each amp card?

Yes, essentially that's it.

> What is the relationship between the nameplate voltage on the amp and
> that on the servo motors.

Not a whole lot. But, you will get the best performance with motors that
can accept most of the bus voltage at the highest speed you expect to
move at.

For an example, let's say you have a 5 TPI screw, and are going to direct
couple the motors to the screw. Let's say you want to reach 120 IPM for
a top speed. 120 IPM x 5 TPI = 600 RPM on the screw, and the motor.
So, a motor that is rated at 6000 RPM at 120 V is not a good choice, as
it will need only about 12 V to do 600 RPM. A motor that is rated at
1000-1800 RPM or so at 75 to 120 V would be a good choice for these
motion specs and your amps.

> If "bus voltage" is 100 VDC does that mean
> I'm hunting for 100 VDC servos. I do understand that the continuous
> wattage rating of the motors should not exceed that of the
> amplifier.

Well, really, that's not such a problem. Going the other way is a bad
idea, as excessive current (even just a short pulse) can demagnetize
the permanent magnets. Motor inductance is a concern, as some
amps are quite picky about having motor inductance within a narrow
range.

> Lastly, I've checked through the archives, but can someone suggest a
> source for Bport-sized PMDC servos w/ tach? Maybe some high stepper
> has a redundant pair for sale?

C&H supply, in Pasadena is great, but they know this stuff, and prices are
fairly high.

Herback & Rademan, in NJ, are good, and have a great deal of good stuff
in their catalogs, http://www.herbach.com/index.html

Burden's Surplus Center, in Lincoln, NE is almost a historic site of
mechanical goodies, I don't think they have a web site, yet.

Jon